An active matrix liquid crystal display (LCD) typically comprises a glass or quartz substrate having formed thereon a plurality of pixel electrodes and switching devices. The pixels are defined by connected gate lines and data lines. Each pixel comprises a storage capacitor and a pixel electrode connected to the switching devices. An LCD employing thin film transistors (TFTs) as the pixel switching devices, provides advantages of low power consumption, thin profile, light weight and low driving voltage. With applications in desktop computer and other monitors, and notebooks, TFT LCDs are presently the most common type of display.
To provide an affordable active matrix LCD, it is desirable to reduce the cost associated with the fabrication of the integrated circuits which drive the pixel TFTs. To this end, low temperature polysilicon (LTPS) TFT LCDs have been developed. In LTPS, an amorphous silicon is deposited onto a substrate and then annealed with laser energy provided, for example, by an excimer laser. The laser annealing process crystallizes the amorphous silicon thereby forming polycrystalline silicon (polysilicon) with large, uniform grains. With LTPS TFT technology, the driver and other related circuits, that are usually located external to the substrate, may be fabricated on a peripheral circuit region of the substrate adjacent to the pixel TFTs (which are fabricated on a pixel region of the substrate).
For an active matrix LCD, the LTPS TFTs of the peripheral circuit region should have high mobility and on-state current characteristics and the LTPS TFTs of the pixel region should have low leakage current characteristics. However, because the polysilicon grains are large, the polysilicon is not conducive to making TFTs with low leakage current characteristics.
Thus, in order to achieve such characteristics, prior art active matrix LCDs employed LDD or offset structures to reduce leakage current of the pixel LTPS TFTs. Such structures, however, undesirably require additional mask and implantation processes and equipment. In addition, these structures reduce the device mobility of the peripheral circuit TFTs.